The invention relates to the detection, enumeration, and identification of replicating cells, especially microbial cells (e.g., bacteria, yeasts, and molds), in medical, industrial, and environmental samples. Microbial culture is the predominant methodology in these markets, because of its many attractive features. The invention addresses the chief drawback of microbial culture—the length of time needed to achieve results—while retaining the beneficial attributes of the method.
Microbial Culture for Detecting and Enumerating Microbes
During the 19th and 20th centuries an understanding emerged concerning the role of bacteria, yeast, and molds in causing infectious diseases and determining the quality of foods and beverages. Early on, a powerful method, microbial culture, was developed for detecting small numbers of microbes. Microbial culture allows simple visual detection of microbes by exploiting their propensity to reproduce in large numbers rapidly. For example, a single bacterial cell, which is much too small to see by eye (about one millionth of a meter), when placed in nutrient broth, can cause the broth to become visibly cloudy in less than 24 hours.
A related microbial culture technique, called microbial enumeration or colony counting, quantifies the number of microbial cells in a sample. The microbial enumeration method, which is based on in situ microbial replication, generally yields one visually detectable “colony” for each microbial cell in the sample. Thus, counting the visible colonies allows microbiologists to determine the number of microbial cells in a sample accurately. To perform microbial enumeration, bacterial cells can be dispersed on the surface of nutrient agar in petri dishes (“agar plates”) and incubated under conditions that permit in situ bacterial replication. The individual, visually undetectable, microbe replicates repeatedly to create a large number of identical daughter microbes at the physical site where the progenitor microbial cell was deposited. The daughter cells remain co-localized (essentially contiguous) with the original cell, so that the cohort of daughter cells (which may grow to tens or hundreds of millions of cells) eventually form a visible colony on the plate.
Electronic methods have been developed for enumerating microbial colonies. Most such methods automate colony counting but do not substantially increase the sensitivity or decrease the time to results compared to traditional enumeration by eye. Colony counters use a variety of optical methods for detecting colonies including detection of intrinsic optical properties of microcolonies (e.g., U.S. Pat. No. 3,493,772; U.S. Pat. No. 3,811,036; U.S. Pat. No. 5,290,701; Arkin, A. P., et al. (1990); Biotechnology (NY) 8: 746-9) and color changes of pH indicator molecules in the matrix surrounding the colonies (U.S. Pat. No. 5,510,246). Methods that use stains or probes to label the colonies have also been developed and will be discussed below.
Microbial culture is a remarkably successful method, as evidenced by the fact that even after more than a century, the method still dominates medical microbiology and quality control testing in industrial microbiology (e.g., pharmaceutical, food, and beverage manufacturing). The method is inexpensive, relatively simple, and ultra-sensitive. The sensitivity of microbial culture can be seen in the common test for foodborne pathogens in ground beef. A single microscopic bacterial pathogen cell can be detected in 25 grams of ground beef using microbial culture. Another advantage of microbial culture is its ability to detect a large range of microbes of medical and industrial significance.
An advantage of in situ bacterial replication is the ability to generate a pure, or clonal, population of cells (called pure cultures, clones, or colonies). A pure culture is a large collection of identical living cells which descend from the same progenitor cell. Pure cultures are required for methods that identify microbes and for determining antibiotic resistance. Medical microbiology relies heavily on pure cultures, since bacterial pathogens are frequently isolated from non-sterile clinical samples (e.g., feces or wounds) along with non-pathogenic bacteria that are likely to be even more numerous than the pathogenic cell. Isolating pure microbial cultures is also important in industrial microbiology. For example, pharmaceutical and cosmetics manufacturers must test their products for the presence of microbial contaminants. Pure cultures of the contaminating microbes are used for microbial identification, which determines whether a production batch must be discarded and aids in investigating the source of the contamination in the industrial process.
TABLE 1Microbial enumeration using microbial cultureAdvantagesultra-sensitivequantitativegenerates pure culturescan detect and enumerate many types of microbes in a single testcan selectively grow microbesonly detects replicating cellsinexpensivesimple and easy to performDisadvantagesslowmanual procedures and analysisnot all microbes are culturable
The ability to culture microbes selectively is an essential tool for microbial identification and for determining resistance and susceptibility to antimicrobial agents such as antibiotics. Selective culture exploits the fact that different microbes require different growth conditions. These differences arise from the fact that strains of microbes differ in their biochemical makeup because of inherent genetic differences. For example, one type of microbe might be able to grow on nutrient medium containing the sugar sorbitol as the sole source of carbon atoms to fuel its growth, while another type of microbe cannot. Selective growth is important in the food industry. For example, a food sample can be scanned for a particular food pathogen, Salmonella, by plating the sample on media that allows Salmonella to grow but not other food microbes.
Similarly, selective culture is used to determine which antibiotic is most effective for killing a bacterial strain isolated from the spinal fluid of a child with bacterial meningitis. A pure bacterial culture (derived from a clonal colony from a nutrient agar plate) is used to inoculate growth medium containing various antibiotics at various concentrations. The optimal antibiotic therapy is determined by monitoring the ability of the microbe to grow in the presence of the various antibiotics. Determining antibiotic resistance and susceptibility by selective growth on the surface of solid nutrient agar medium is another common approach. For example, in the Kirby-Bauer method, small filter disks impregnated with different antibiotics are placed on the surface of nutrient agar plates coated with a pure culture of bacteria from a clinical sample. A gradient of antibiotic diffuses radially outward from the filter. Bacteria that are resistant to high levels of the antibiotic grow up to the edge of the filter. However, bacteria that are very sensitive to the antibiotic can not grow unless they are far from the edge of the filter. After incubating the plates (usually for one or two days) a microbiologist determines the level of resistance to an antibiotic by measuring the thickness of the growth-free ring or zone around the filter. A related method, the “E” test (Hardy diagnostics), uses a rectangular strip that is impregnated with a gradient of antibiotic. The level of bacterial resistance is determined by measuring the point on the strip with the highest antibiotic concentration next to which the bacteria continue to replicate.
The most serious drawback of microbial culture is that it is slow—it takes time to generate the number of cells required for visual detection. The long growth period required for microbial culture is a significant problem in both healthcare and industry. For example, because it requires days to culture and identify the microbe causing a patient's blood infection, a patient with a fungal blood infection could die before anti-fungal therapy is even begun. Some infectious agents, such as the bacterium that causes tuberculosis, generally require weeks to grow in culture. The long time required for detecting M. tuberculosis can result in a patient with tuberculosis infecting many others with the highly contagious disease or the costly quarantine of patients who do not have tuberculosis.
In food manufacture, long testing cycles can increase food spoilage or result in moving inadequately tested material through subsequent processing steps. Slow microbial culture also adversely impacts the production of biopharmaceuticals and vaccines. In these applications, the manufacturing process often requires pooling of batches. Because of long microbial culture testing cycles and the need to move material through the manufacturing process, contaminated batches are sometimes not detected until after a batch pooling step. If it is subsequently found that a contaminated batch was combined with uncontaminated batches, the whole pool of combined batches must be discarded.
Other disadvantages of microbial culture, such as tedious manual procedures and inability to culture some microbes, are considered less problematic than the long time required. For example, manual methods for microbial enumeration predominate, even though instruments for automated plating and analysis have been introduced. Most types of microbes found in the environment cannot be grown in the laboratory. However, such microbes are often not harmful to humans or are destroyed in industrial manufacturing processes and are therefore ignored for most applications. However, several important exceptions of critical medical importance include hard or impossible to culture bacteria such as Chlamydia, strains of which can cause sexually transmitted disease and pneumonia. Fortunately, alternative culture-independent methods are available in these cases (see below).
Rapid Microbial Culture Enumeration Methods
A number of microbial culture methods for more rapid microbial enumeration have been developed. One rapid microbial culture method deposits bacterial cells on microscope slides coated with nutrient medium. Using microscopic examination, microbial growth can be detected much earlier than with the naked eye, since microscopes can detect microcolonies resulting from a small number of cell divisions. However, this method is not effective for testing large samples containing low numbers of microbial cells, because only a very small volume of sample can be observed in a microscopic field of view. The low sensitivity of microscopic methods generally limits their usefulness to samples containing more than ten thousand bacterial cells per milliliter—these methods are much less sensitive than traditional microbial culture.
The advent of electronic imaging systems has led to the development of numerous automatic “colony counters.” Although, most of these counters are designed to aid the user by automating the colony counting process and do not decrease the time to result, some systems have demonstrated the ability to detect colonies before they are large enough to be seen easily by eye. For example, the Colifast Rapid Microcolony Counter (Colifast) can detect small fluorescently labeled colonies of coliform bacteria hours before they can be seen by eye. The Colifast system achieves enhanced detection by using a fluorogenic compound (a substance that is not fluorescent until metabolized by coliform bacteria) included in the nutrient agar media.
A system for rapid enumeration of microbial colonies using bioluminescent labeling has recently been commercialized. The MicroStar system (Millipore) uses the cellular ATP in microcolonies to generate light via the action of applied luciferase enzyme and substrates. The method reduces time to detection substantially. The MicroStar imaging system has also been used in conjunction with labeled probes to identify specific bacteria (Stender, H., et al. J Microbiol Methods 46: 69-75 (2001)). A drawback of the system is that the detection method kills the microbes, precluding isolation of pure cultures from the colonies. The system also requires an expensive image intensifier module.
An instant film-based method for detecting microcolonies containing specific bacteria has been developed by Boston Probes (Perry-O-Keefe, H., et al. Journal of Applied Microbiology 90: 180-9 (2001)). Microbial microcolonies on membranes are labeled using microbe-specific PNA probes tagged with an enzyme capable of generating a chemiluminescent signal. The membranes are then placed on X-ray or instant-film for imaging. The method is limited to scanning for a particular microbe in one experiment. A similar method uses fluorescently labeled PNA probes and an array scanner (Stender, H., et al. Journal of Microbiological Methods 45: 31-9 (2001)). These approaches require substantially more expertise than traditional culture methods.
Rapid Microbial Enumeration without Microbial Culture
The fastest methods for microbial enumeration forgo microbial culture. Medical and industrial microbiologists are generally interested only in enumerating viable microbes—only living microbes are capable of replicating during microbial culture. Therefore, to be most effective, methods that detect individual cells without reliance on cellular replication must distinguish living from dead microbes by using physiological surrogates for cellular replication (e.g., Nebe-von-Caron, G., et al., J Microbiol Methods 42: 97-114., 2000; Mignon-Godefroy, K., et al., Cytometry 27: 336-44, 1997). Cells are stained with dyes that measure a biochemical property that is generally correlated with the ability to replicate (e.g., esterase activity or biochemical respiration). Validating and instituting surrogate methods have been problematic since samples that are known to meet regulatory standards and that are scored as sterile using traditional plate culturing methods often have thousands of cells that score positive for the surrogate biochemical activity.
An example of a system that directly detects viable cells is the ScanRDI system (Chemunex). ScanRDI enumerates microbial cells that are stained with a fluorogenic esterase substrate using laser scanning technology (U.S. Pat. No. 5,663,057; Mignon-Godefroy, K., et al., Cytometry 27: 336-44, 1997). A laser-scanning system (including an optical collection system using photomultiplier tubes (PMTs)) captures an image of the filter and can detect individual labeled cells. The system illuminates and queries a microscopic area (generally 4-14 μm) but scans the beam progressively so as to cover a macroscopic area (e.g., a 25 mm diameter circle). The system is designed to detect cells with intact membranes and active esterase enzyme. There is a correlation between the numbers of such cells and the number of cells that can form colonies on growth medium. However, this approach often results in substantial “overcounting”—i.e., higher numbers of cells than are detected by traditional culture (Costanzo, S., et al. (2002). PDA Journal of Pharmaceutical Science and Technology 56: 206-219). Another disadvantage of the ScanRDI system is that it kills the microbes during the staining process precluding generation of pure cultures from the detected microbes. Finally, laser scanning systems for cellular enumeration are complex and expensive (hundreds of thousands of dollars) making them difficult to justify for routine microbiological applications. Other laser scanning systems have also been commercialized (Miraglia, S., et al., J Biomol Screen 4: 193-204, 1999; Tibbe, A. G., et al., Nat Biotechnol 17: 1210-3, 1999; Kamentsky, L., 2001, Laser Scanning Cytometry. In Cytometry, Z. Darzynkiewicz, H. Crissman and J. Robinsnon, eds. Methods in Cell Biology Vol. 63, Part A, 3rd ed, Series Eds. L. Wilson and P. Matsudaira. (San Diego: Academic Press)).
Flow cytometry is another powerful method that can rapidly enumerate microbes without relying on cellular replication (Alvarez-Barrientos, A., et al., Clin Microbiol Rev 13: 167-195, 2000). Individual organisms or particles are forced to flow through a narrow channel, one at a time, past a laser beam. Besides enumeration, information about size/shape and composition is gathered by analyzing the fluorescence emission and light scattering caused by the organisms. Thousands of individual cells or particles can be analyzed per minute. Pathogens can by identified using flow cytometry by binding fluorescently labeled species-specific antibodies or nucleic acid probes to fixed organisms (Alvarez-Barrientos, 2000, supra).
Pathogens can by identified using flow cytometry by binding fluorescently labeled species-specific antibodies or nucleic acid probes to fixed organisms (Alvarez-Barrientos, 2000, supra). Individual cells of one particular type are usually the targets. Flow cytometric methods have been used more extensively for quantitatively detecting particular cell types on the basis of the ability to bind labeled probes, usually either antibodies or nucleic acids. For example, flow cytometry is used to quantify the population sizes of classes of lymphocytes in patients with AIDS. Flow cytometry is a more complex and expensive method than traditional culture. Although faster than traditional culture, flow cytometry does not have a comparable limit of detection to the traditional method. Traditional microbial culture can detect one bacterial cell in 0.1 liter of water, while flow cytometry is most effective when there at levels that are many thousands of times higher than that. Furthermore, microbial targets are often killed by the staining methods used for detection, eliminating the ability to produce pure cultures.
Using microscopic imaging to visualize and enumerate microorganisms directly can be rapid and relatively simple to perform (Amann, R. I., et al., Microbiological Reviews 59: 143-69, 1995). Direct fluorescent assays (DFA) in which a fluorescently labeled antibody reacts with a fixed sample is a common method in clinical diagnostics laboratories. For example, specimens suspected of containing bacterial agents are routinely stained with Gram stain. Similarly, to test for M. tuberculosis, samples are subjected to acid fast staining. The drawback of this technique is that it is many thousands of times less sensitive than microbial culture. The low sensitivity is due to the small fields visualized at high magnification. Only at high target cell concentrations are small fields likely to contain a target cell. Thus, for example, reliable identification of bacterial pathogens in sputum using fluorescent in situ hybridization requires titers of about 4×105 cells/ml or more. Clinical samples obtained in common medically significant infections may contain fewer than 100 cells/ml—a concentration that is not nearly high enough to expect to find a cell in a high power microscopic field.
A system that does have the sensitivity to detect single bacterial cells using large area non-magnified imaging has been developed by researchers at Hamamatsu Corporation (Masuko, M., et al., FEMS Microbiol Lett 67: 231-8, 1991; Masuko, M., et al., FEMS Microbiol Lett 65: 287-90, 1991; Yasui, T., et al., Appl Environ Microbiol 63: 4528-33, 1997). Large area imaging of individual microscopic target cells is accomplished using an ultrasensitive photon-counting CCD camera coupled to a fiber optic system, image intensifier, and Image-Processor. A disadvantage of this system is the great expense incurred because of the incorporation of the image intensifier and associated optics. Furthermore, unlike microbial culture methods, the system can not detect any microbe, distinguish between living and dead microbes, or generate pure cultures.
Rapid Microbial Enumeration by Quantifying Molecular Constituents of Cells
Numerous methods for detecting and identifying microbes based on their molecular constituents have been developed in the last half-century. Although some of these methods are substantially faster than microbial culture, none offers all of the features of culture that are critical to microbiologists. For example, although numerous immunoassays for microbes have been commercialized, this technique is not inherently quantitative, is much less sensitive than microbial culture, and is not as powerful as culture for detecting many types of microbes in a single test. Or, as another example, nucleic acid amplification methods can be as sensitive as microbial culture, but they do not distinguish between living and non-living cells and can not deliver pure cultures for antibiotic susceptibility testing. Methods for biochemical analysis (e.g., of fatty acids, nucleic acids, or proteins) using electrophoresis, mass spectroscopy, and chromatography can be powerful for microbial identification, but such methods are usually inappropriate for microbial enumeration and are generally too expensive and complex for routine microbial diagnostics.
Unmet Needs for Microbial Enumeration
In summary, current microbial enumeration testing is dominated by microbial culture. Microbial culture has the important advantages of being simple, ultra-sensitive, inexpensive, and quantitative but has the significant drawback of being slow. The long time required for results has major costs in healthcare and in manufacturing. More rapid methods have been developed, but while improving the time to results, they have sacrificed one or more of the critical advantages of microbial culture.
Thus, there is need for a test that is faster than traditional microbial culture but that retains the key benefits of the traditional method.